The present invention relates to a sheet handling apparatus and a method of sheet handling and, more particularly, to such apparatus and method for transporting cut sheet paper along a predetermined path and for selectively directing individual sheets to a second, divergent path. Such apparatus and method may find particular application in printing systems, where the sheets are selectively transported to various components of the printing device. At various points along the path, it may be necessary or desirable to provide divergent paths for removing damaged sheets, routing specific sheets to alternate outputs, returning sheets to a printing station for printing on a second side, sorting or re-ordering the sheets, or the like.
There are several methods presently known whereby a sheet may be diverted mechanically from a first path into a second path. Diverting plates may be moved into the path of the sheets to deflect the sheets out of their original path and into a second path. Alternatively, rollers may be moved into contact with the sheets so as to feed the sheets into the divergent path. Such methods may not be appropriate, however, where the sheets are travelling at relatively high speeds. It is extremely difficult to move a high mass mechanical deflection system quickly enough so as to divert a single, selected sheet without affecting the normal flow of the preceding and following sheets through the sheet path.
It is also well known to transport sheets by means of a hollow drum having openings along its surface and a partial vacuum applied to its interior. The drum is rotated and one or more sheets are fed onto the drum, adhering to its surface due to the partial vacuum applied to the sheets through the openings. The sheets are then carried by the rotation of the drum.
The sheets may be removed from the drum by any of several known methods. Mechanical means are known whereby an actuator mechanism located adjacent the drum pivots several deflection fingers into grooves defined by the surface of the drum, such that the leading edges of the fingers are below the drum surface. The fingers effectively peel the sheets from the drum surface. Such a system is disadvantageous for use in a high speed sheet handling apparatus, since the mechanism must be pivoted very quickly. Moreover, precise recognition of the leading and trailing edges of the sheets is required in order to time the actuation of the mechanism.
In U.S. Pat. No. 1,838,200, issued Dec. 29, 1931, to Tomtlund, the openings in a vacuum drum are connected by a series of lateral passageways just under the exterior surface of the drum. By use of fixed baffles placed in the ends of the drum, the passageways communicate with the partially evacuated interior of the drum during the portion of the drum's rotation in which sheets are fed onto the drum. During that portion of the drum's rotation in which the sheets are to be released, the passageway communicates with the atmosphere. With no vacuum exerted upon the sheets, the sheets are released and removed from the drum by their own weight.
In U.S. Pat. No. 3,669,446, issued June 13, 1972, to Derc et al, a baffle plate is mounted within a vacuum drum adjacent the inner surface thereof at the point where a sheet is to leave the surface of the drum. The plate, by blocking the openings in the drum surface adjacent thereto, prevents the vacuum from reaching the sheet, and the sheet is thereby released from the drum.
Neither of these methods is suitable for selectively releasing a particular sheet from a vacuum drum, and diverting it to an alternate path. In the Tomtlund '200 patent, the means for disconnecting the passageways from the partial vacuum is an integral part of the drum and its support, and operates in a noncontrollable fashion such that selective sheet release may not be obtained. In the Derc '446 patent, the plate is fixed in place, such that every sheet is released from the drum at the same point along the drum's rotation.
In U.S. Pat. No. 4,216,954, issued Aug. 2, 1980, to Kwasnitza, an apparatus is disclosed for selectively diverting a particular sheet to an alternate path. A rotatable hollow drum having openings along its surface is positioned at the juncture of two sheet paths. The paths are formed by endless conveyor belts, some of which pass over the drum, thereby rotating it. A rotatable chamber is located in the interior of the drum, communicating with its inner surface, and may be selectively connected to either a partial vacuum source or a source of slightly compressed air. The chamber is rotated so that it meets the leading edge of each successive incoming sheet as the sheet approaches the drum. If the sheet is to continue in the first path, the chamber is connected to the source of slightly compressed air, so that air emitted through the openings in the drum will urge the leading edge of the sheet away from the drum, thereby causing the sheet to remain in the first path. If the sheet is to be diverted to the second path, the chamber is connected to the partial vacuum source. The vacuum is applied to the leading edge of the sheet through the openings in the drum, thereby carrying the sheet along the drum surface to the entrance to the second path.
While the device disclosed in the Kwasnitza '954 patent does provide for diverting selected sheets to an alternate path, there are several disadvantages inherent in this device. Most apparent is the relative complexity of the apparatus, requiring both a rotating drum and a rotating chamber. Further, sources of both compressed air and partial vacuum must be provided. Additionally, the rotational speed of the chamber must be precisely timed so that the chamber meets the leading edge of each sheet as the edge approaches the drum, and means must be provided to vary the speed of the chamber in order to handle sheets of different sizes.
It is seen, therefore, that a relatively simple means of diverting selected sheets from a transport path into an alternate path is needed. Such a means should provide diversion at relatively high speed without interruption to the normal flow of preceding and succeeding sheets, and should be operable regardless of the size of the sheets or spacing between successive sheets. Moreover, such a means should divert selected sheets with minimal risk of damage to those sheets.